Abstract

Rhodobacter sphaeroides 2.4.1 exposed to selenate or selenite produced
volatile selenium compounds. Total amounts of dimethyl selenide, dimethyl
diselenide, dimethyl sulfide and dimethyl disulfide in culture medium and
headspace were determined. The highest selenate volatilization occurred
in the late stationary phase of growth. However, cultures deprived of light
in the stationary phase of growth produced much less of the volatile organo-selenium
compounds. Lower culture pHs increased the rate of selenium volatilization.
Low sulfate concentration limited biomass production and selenium volatilization;
high sulfate concentrations had an enhancing effect on the release of organo-selenium
compounds. Cultures of R.Êsphaeroides reacted very differently to
amendments with increasing amounts of selenate and selenite. Only small
amounts of selenite were volatilized; meanwhile high amounts of methylated
selenides were found in selenate poisoned cultures. Keywords: Rhodobacter
sphaeroides, selenate, selenite, reduction, methylation.

In a comparison of samples incubated in the light or in the dark, this
figure compares the production of dimethyl selenide (CH3SeCH3),
dimethyl selenenyl sulfide (CH3SeSCH3), and dimethyl
diselenide (CH3SeSeCH3) in cultures of Rhodobacter
sphaeroides 2.4.1 grown on a minimal medium at about 28 degrees C.
Clearly the samples grown in light produced more organoselenides even though,
compared to the dark samples, their cell populations were not significantly
different. This strongly suggest that reduction and methylation of selenate
in these cultures was favored by light driven processes.

The effects on bacterial growth of a range of culture pH was investigated
in this second figure.
While biomass production appears to maximize at a pH of about 6 to 6.5
depending on whether or not selenate has been added, dimethyl selenide
production was not a simple function of pH.

Three days after adding 1 mM selenate to the following cultures, the
effect of different carbon sources were
investigated. In general, the higher the redox potential of a carbon
source (more oxidized), the higher the concentration of the organoselenides
determined in the bacterial headspace of cultures grown on that substrate.

Finally, the variation of amounts of selenate and selenite added to
this microorganism grown on a single carbon source (20 mM succinate) showed
interesting
effects on biomass production and headspace production of organoselenides
determined after 7 days of photosynthetic growth. Selenate at levels of
6 mM and more led to a decrease in biomass production in poisoned cultures
(Figure 4A). The highest amounts of dimethyl selenide were found in the
headspace above cultures amended with 6 to 10 mM selenate; the highest
yield of 0.16% (selenium contained in dimethyl selenide and dimethyl diselenide)
occurred when the cultures were amended with 6 mM selenate.